My invention is adaptable for use in an automatic transmission of the kind as shown, for example, in U.S. Pat. Nos. 3,714,836 and 3,497,043, which are assigned to the assignee of this invention.
In the case of the transmission shown in the '836 patent, a hydrokinetic torque converter has an impeller connected to an internal combustion engine in a vehicle driveline and a turbine adapted to be connected to torque input elements of the gearing through friction clutches, one clutch being effective for forward drive and the other clutch being effective for reverse drive. When the transmission is in neutral, both clutches are disengaged. Thus, the converter effectively then is disconnected from the planetary gearing.
Upon engagement of the clutch to establish forward drive operation, the turbine is connected directly to an input element of a planetary gear unit through the forward clutch. A reaction brake on the carrier for the gear unit establishes a reaction point as a forward-driving, low-speed ratio torque flow path is established between the turbine and the power output shaft of the transmission.
The neutral condition is established by a manual valve which interrupts the flow of fluid from the pressure source to the clutches when the manual valve is in the neutral position. If the manual valve is shifted from the neutral position to the forward drive position, clutch actuating pressure is distributed to the forward clutch. Similarly, if the manual valve is shifted from the neutral position to the reverse drive position, the reverse clutch becomes engaged, thereby establishing a driving connection between the turbine and a different torque input element of the gearing as a reverse torque flow path is established between the turbine and the power output shaft of the transmission.
The transmission of the '043 patent includes four forward driving ratios and a single reverse ratio in contrast to the transmission of the '836 patent, which has three forward driving ratios. Like the transmission of the '836 patent, the transmission of the '043 patent includes a manual valve adapted to distribute pressure to a forward clutch when the manual valve is moved to the forward drive position from the neutral position. This establishes a driving connection between the turbine of a hydrokinetic torque converter and the input ring gear of a first of three planetary gear sets. If the manual valve is moved to the reverse position from the neutral position, the reverse clutch is engaged, thus establishing a driving connection between the turbine and the sun gear of a second gear set.
In each of the transmission arrangements of the reference patents, the converter turbine is disconnected from the power output shaft and from the traction wheels of the vehicle when the manual valve is in the neutral (or park) position. This is due to the fact that the forward clutch and the reverse clutch then are disengaged, thereby interrupting the torque flow path from the turbine. The turbine, in effect, is unloaded under these conditions. When the manual valve is moved from the neutral or park position to the drive position, the forward clutch is pressurized. As capacity builds up in the forward clutch, the turbine thus becomes coupled to the traction wheels through the planetary gearing.
Provision is made for softening the engagement of the forward clutch or the reverse clutch. If the pressure is allowed to rise in the clutches too rapidly, that may cause a so-called clutch engagement bump. On the other hand, if the clutch pressure in the reverse clutch or the forward clutch is allowed to buildup too slowly, the transmission engagement may be delayed. It is possible then for an engine speed-up to occur as the driver depresses the throttle before the clutch is fully engaged. This creates an inertia force that is felt as a shock by the driver of the vehicle upon late engagement of the clutch.
I am aware of various attempts to eliminate clutch engagement shock when the manual valve is shifted from the park or neutral position to either the reverse drive position or the forward drive position. An example of a control mechanism that compensates for this shock is shown in U.S. Pat. No. 4,966,050, which also is assigned to the assignee of this invention. The transmission control of the '050 patent includes a valve member located between a friction clutch associated with planetary gearing and a pressure source, which usually is a positive displacement pump. The pressure of the pump is regulated by a main pressure regulator valve. An overdrive clutch serves as an accumulator which communicates through an orifice with a line pressure passage on the downstream side of a cushioning valve. The pressure in the overdrive clutch or the so-called accumulator acts on the cushioning valve to effect a gradual pressure buildup in the transmission friction clutch.
Another prior art arrangement for controlling harshness in the engagement of a transmission friction clutch is taught by U.S. Pat. No. 4,265,346, which also is assigned to the assignee of this invention. That patent discloses a clutch engagement control valve located in a clutch feed passage in parallel disposition with respect to the servo for the clutch. The clutch control valve regulates the pressure in the clutch following a command for clutch engagement by progressively restricting the control pressure feed flow path to the clutch in response to a pressure buildup in the clutch. The pressure buildup is caused to act on the valve to oppose a valve spring acting on the valve. The force of the spring is supplemented by the force of an accumulator pressure in the accumulator mechanism situated in fluid communication with the valve chamber occupied by the spring so that the accumulator pressure force complements the force of the spring. A torque signal is distributed to the accumulator to raise the effective accumulator pressure level, depending upon the magnitude of the torque distributed to the input side of the transmission mechanism from the engine.